1. Field of the Invention
This invention relates to a digital data playback system used for reproduction of digital data recorded on a recording medium, such as a magnetic tape, and more particularly to a time base correcting device for correcting any error in the playback timing during reproduction of recorded digital data.
2. Description of the Prior Art
In the course of recording and reproducing data on and from a recording medium, the time base of reproduced data is generally subjected to a variation due to factors including expansion and contraction of the recording medium and speed variation. Therefore, it is necessary to strictly regulate the time interval of the data, and, for this purpose, means are required for correcting such a time base error of the reproduced data. For example, in a digital picture recording and reproducing system, a picture is divided into a plurality of blocks B.sub.1 to B.sub.59, and each of the blocks is further divided into sub-blocks SB.sub.1 to SB.sub.20 as shown in FIG. 1. In the recoding mode, a digital picture data of a predetermined length [represented generally by a sub-block (Bi-Sbj) herein] is recorded as a unit on a recording medium such as a magnetic tape, and number data including a block number Bi and a sub-block number SBj is inserted in the head of each sub-block to form each record, as shown in FIG. 2. In the playback mode, the digital picture data of each sub-block is written once in a memory at an address corresponding to the affixed number data (the block number Bi and sub-block number SBj), and the picture data sequentially written in the memory is read out in synchronism with a stabilized system clock applied from a time base correcting device, so that the time base of the picture data read out from the memory can be corected, by means of the time base correcting device.
However, when, for example, degradation of the reproduced signal quality or dropout of data occurs due to the presence of a scar on the tape or attachment of dust to the tape, a code error appears in the reproduced digital picture data. When such a code error appears in the part of the number data (the block number Bi and sub-block number SBj) of the records, the reproduced digital picture data is written in the memory at incorrect addresses regardless of the function of the time base correcting device. In such a case, the read-out sequence becomes out of order resulting in appearance of a serious defect on the reproduced picture. Suppose, for example, that a picture is divided into ten blocks B.sub.1 to B.sub.10 as shown in FIG. 3(a), and errors occur in the blocks B.sub.3 and B.sub.5, resulting in relative displacement of these blocks B.sub.3 and B.sub.5. In such a case, an incorrect picture as shown in FIG. 3(b) will be reproduced.
Japanese Unexamined Patent Publication No. 55-80868 (1980) proposes a solution to the problem described above. According to the disclosure of the cited publication, a predetermined synchronization pattern is used in lieu of the address data such as the sub-block number, and the write position in the memory is specified by a count of a counter responding to detection of such a sync pattern. However, the proposed method is also defective, in that, once proper detection of the sync pattern fails due to, for example, the presence of dropout, the count of the counter does not indicate the correct write-in position any more, and appearance of disorder on the reproduced picture results.
Japanese Unexamined Patent Publication No. 58-99080 (1983) proposes another solution to the aforementioned problem. The method disclosed in the cited publication comprises recording an error detection and correction code together with inserted number data, estimating succeeding number data on the basis of preceding number data used for specifying the writein position, and using the read-out number data in the playback mode when no error is detected or when a detected error is correctable, but using the estimated number data when the detected error is not correctable. However, the error detection and correction code requires extra bits, and its ability to effect error detection and correction to completely eliminate the possibility of overlooking errors in limited. An attempt to enhance the error detection rate leads necessarily to a great increase in the number of required bits. Especially, when the error is attributable to the presence of a burst, for example, or a dropout, not much can be expected of the error detection by the use of redundant bits.